DWCNTs are unique nanostructures consisting of two concentric single-walled carbon nanotubes (SWCNTs), which are sheets of carbon just one atom thick that have been rolled up into a tube with a diameter of about 1 nm. DWCNTs can exist in four different electronic types, defined by the constituent inner and outer walls, which can be metallic (M) or semiconducting (S). This gives rise to four possible electronic combinations, known as M@M, S@M, M@S and S@S, for inner@outer wall.

“The electronic and physical coupling between the two walls should lead to new properties not seen in SWCNTs, so DWCNTs must thus be considered as a whole new type of nanostructure, rather than simply the combination of two individual SWCNTs,” says team leader Benjamin Flavel of the Karlsruhe Institute of Technology. “These new properties could open exciting avenues in carbon science in the future, such as the existence of superconductivity in M@M DWCNTs, for example.”

Test bed and novel applications

“DWCNTs are also the simplest form of multiwalled CNTs and thus provide a test bed to study the physics of two co-axially aligned interacting carbon walls using techniques like Raman spectroscopy, and it will be interesting to see what new properties we will discover in these materials in the future.”

DWCNTs that are either purely semiconducting (S@S) could be ideal in applications like CNT-FETs for use in chemical and biological sensors, in which the outer wall acts as a scaffold that can be modified with receptor molecules or which alternatively acts to shield the inner wall from the external environment. Until now, it had only been possible to separate DWCNTs according to their outer-wall electron-type, however.

New approach makes use of a surfactant

Flavel and colleagues have now succeeded in sorting DWCNTs by semiconducting and metallic inner- and outer-wall electronic types for the first time using a technique that makes use of surfactant – not only to disperse the raw-unsorted DWCNTs in an aqueous solution but also then to separate them. This approach has proved itself to be extremely successful for SWCNTs in the past.

“The surfactant forms a micelle around the CNTs and the way it covers the tubes and the shape it forms around them depends on the CNT electronic type,” explains Flavel. “We then exploit the difference in the surfactant structure to tailor the interaction of the wrapped CNTs with other media, such as gel columns, and so separate them by type. The result is purified fractions of M@M, S@M, M@S and S@S electronic types.”

Becoming more sensitive to the inner wall

“Since the surfactant shell is only in physical contact with the outermost layer of a DWCNT, the shape and structure of the shell was in the past thought to be entirely defined by this outer wall. However, in reality there is an electronic interaction between the two walls, so we used this fact to develop experimental conditions that allow us to become more sensitive to the inner wall, despite us only being in physical contact with outer wall,” he tells nanotechweb.org.

The team, which includes researchers from the Freie Universität Berlin, the Helmholtz Institute in Ulm and the Technische Universität Darmstadt, is now attempting to prepare DWCNTs with defined chirality. (The chirality, or handedness, of a nanotube is the relative orientation of the hexagonal lattice in which the atoms in the carbon sheet of the nanotube are arranged to the axis of the tube, and it determines whether the tube is a metal or a semiconductor). “This has already been shown to be possible for SWCNTs and now that we can access the inner wall type in DWCNTs, there is no reason to believe why it will not be possible in DWCNTs,” says Flavel. “The large diameter of the DWCNTs and the very small difference in the way the surfactant shell wraps around different DWCNT electronic types is going to make this challenging, however.”

The research is detailed in Nature Nanotechnology doi:10.1038/nnano.2017.207.